Batteries used in Implantable Medical Devices (IMDs), such as cardiac pacemakers and Implantable Cardioverter Defibrillators (ICDs), are required to meet high quality and performance specifications and reliability. They need to have high energy density, high rate capability and long shelf life. Since replacement of the battery of an IMD means that the patient must undergo surgery, batteries for IMDs must have a long service life. Improvement to the reliability, performance, and lifetime of such batteries is highly desirable.
ICDs treat ventricular fibrillation, also known as sudden cardiac death. Ventricular fibrillation is characterized by rapid, erratic contraction of the heart resulting in little or no pumping of blood and is generally a fatal condition. An ICD delivers a high-energy pulse (typically up to 35 J) to the heart within seconds of detecting ventricular fibrillation. Minimizing the time a patient remains in fibrillation is an important goal of this therapy. To deliver this life-saving therapy, the ICD battery charges a capacitor to a desired energy level in as short a time as possible, and the capacitor is subsequently discharged through the heart. Because prompt therapy is desirable, the capacitor charge-time, typically in the range of 5 to 15 seconds, is a measure of device performance.
Silver Vanadium Oxide (SVO) batteries are commonly used in medical devices, because they provide a very high energy output rate required by ICDs. The battery consists of multiple cathode (SVO) layers and corresponding anode (lithium metal) layers.
Lithium dendrite formation and resulting internal shorting is one of the major failure modes for ICD batteries. Dendrite formation can result in a short circuit when a bridge is made between an anodic surface and a cathodic surface. Lithium dendrite formation mechanisms are not well controlled through chemistry or geometry changes. Insulation of the active elements in the battery is currently the only method available to mitigate early battery depletion due to a short from lithium dendrite formation. But existing insulation structures have shortcomings.
Insulation joints used in the industry consist of either a simple overlapping fit between insulating parts or an interference joint as described in U.S. Pat. No. 9,281,507. These types of joints exhibit fit variations that result from component tolerances and manufacturing variations, and can leave openings for dendrites to form, allowing for a shorting failure to occur. What is needed is a battery construction that is more resistant to the formation of lithium dendrites in undesirable places.